Binders or binder systems for foundry cores and molds are well known. In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern, and then cured with the use of catalysts and/or heat to a solid, cured state.
Resin binders used in the production of foundry molds and cores ar often cured at high temperatures to achieve the fast-curing cycles required in foundries. However, in recent years, resin binders have been developed which cure at low temperatures, to avoid the need for high-temperature curing operations which have higher energy requirements and which often result in the production of undesirable fumes.
One group of processes which do not require heating in order to achieve curing of the resin binder are referred to as "cold-box" processes. In such processes, the binder components are coated on the aggregate material, such as sand, and the material is blown into a box of the desired shape. Curing of the binder is carried out by passing a gaseous catalyst or hardener at ambient temperatures through the molded resin-coated material.
One such "cold-box" process employs an aqueous alkaline solution of a phenolic resole resin as the binder. This binder is cured by passing the volatile ester through the molded resin-coated material. The process is described in detail in U.S. Pat. No. 4,468,359 (Re. 32,720) which is incorporated herein by reference in its entirety.
The ester cured process is superior to some of the earlier processes from an environmental standpoint. However, the tensile strengths of the cores made by this process tend to be somewhat lower than those prepared by other "cold-box" processes.
We have now discovered that certain modified benzylic ether resole resins, not hitherto used with ester-cured phenolic binder systems, speed the rate of cure of such systems, giving molds and cores with higher initial tensile strength. This reduces breakage of the cores when they are first removed from the core boxes. We have also discovered that if certain additives are used with these resins they give cores and molds of even greater tensile strength.
It is known that benzylic ether resins can be prepared by the reaction of an aldehyde with phenol in the presence of a divalent metal ion catalyst, U.S. Pat. No. 3,485,797. Their use in making polyurethane binders for foundry cores and molds is well known. More recently, in U.S. Pat. No. 4,668,759, a modified phenolic resole resin was prepared by partial reaction of an aldehyde with phenol under strongly basic conditions followed by further reaction at a pH below 7 in the presence of a catalyst which promotes formation of benzylic ether bridges. The presently-disclosed resins differ from both of the foregoing products. The new resins are prepared by partial reaction of the aldehyde and phenol in the presence of a benzylic ether promoting catalyst and then completion of the reaction under alkaline conditions.